The purpose of the proposed study is to determine whether there is a relationship between the efficiency of communication between the left and right cerebral hemispheres, via the corpus callosum, and the ability to sustain attention over an extended period of time, in normal adults. Previous research has suggested such a relationship in normal children, and in clinical populations with impaired callosal transfer. In addition, differences between the left and right hemispheres in performance on a sustained attention task will be assessed. The efficiency of interhemispheric communication will be measured on two separate tasks. A visual tachistoscopic task that requires subjects to compare two stimuli when one is presented to each hemisphere will measure the efficiency of the posterior callosum. A motor task that requires coordinated movement of the left and right hands will assess the efficiency of the anterior callosum. Sustained attention will be measured on a simple reaction time vigilance task that requires subjects to detect infrequently-occurring targets over a period of time. Sustained attention can be defined either as a decline in performance (increase in reaction time or proportion of targets missed) from the beginning of the task to the end, or as the difference in detection of targets presented after long and short interstimulus intervals. Differences between the left and right hemispheres in sustaining attention will be assessed on a similar vigilance task on which the targets occur in either the left or right visual fields, rather than in the center. Correlations will be calculated between performance on all of these tasks. It is expected that adults who exhibit greater callosal efficiency will perform better on the vigilance task. It is also expected that the lateralized vigilance task will show that the right hemisphere can maintain accurate performance over a longer period of time than the left. This study will help to shed light on the function of the corpus callosum in normal adults, and will suggest a possible mechanism behind impaired attention in adult clinical populations.